A growing number of persistent organic micropollutants such as pharmaceuticals and personal care products (PPCPs), endocrine disrupting compounds (EDCs), pesticides, and herbicides are frequently observed in natural and treated water. These contaminants are recalcitrant to conventional water and wastewater treatment and may pose risks to human and ecological systems even at very low concentrations. Persistent and emerging waterborne, foodborne, and airborne pathogens can cause the spread of infectious diseases, and their control is important to protect the public health. Photocatalysis is a promising advanced oxidation process (AOP) for the degradation or mineralization of persistent organic micropollutants and the inactivation of pathogens because it activates O2 and/or H2O at ambient conditions to generate reactive oxygen species (ROS; e.g., .OH, O2−./HO2., H2O2, and 1O2) in situ. Photocatalysis also eliminates the hurdles in the storage, handling, and transportation of oxidants or disinfectants, and potentially uses renewable solar energy or indoor lighting for water purification and antimicrobial applications.
Recently, graphitic carbon nitride (g-C3N4) has emerged as a visible-light-responsive photocatalyst with tunable band gaps of 1.8-2.7 eV that allow the harvesting of visible light up to 460-698 nm (potentially utilizing 13-49% of solar energy, though photocatalytic activity may be reduced at a longer wavelength). g-C3N4 is made from earth-abundant, inexpensive carbon and nitrogen containing precursors (e.g., urea and melamine), is biocompatible with no reported toxicity, is resistant to photo-corrosion, and remains chemically stable in harsh environments.
Supramolecular preassembly of triazine precursors has become an attractive approach to tailor the properties and reactivity of g-C3N4. The supramolecular approach is more environmentally benign and sustainable compared to widely used hard-templating with nanosilica because no toxic or corrosive chemicals are involved (e.g., HF or NH4HF2 for the post-removal of silica and pore generation). Cyanuric acid has been applied with melamine because they can interact with each other by forming hydrogen bonds, producing a highly stable supramolecule as the precursor of g-C3N4. Cyanuric acid is less thermally stable than melamine and decomposes into gases at an elevated temperature, which may create a porous structure of g-C3N4 with an increased surface area and charge separation.
The molecular structure of g-C3N4 has also been altered with metal and/or nonmetal dopants, or nanoparticles to improve charge separation and visible-light utilization. Such methodologies are disadvantageous, however, because metal dopants or nanoparticles are more expensive (e.g., noble metal loading) and may leach or be deactivated in a complex environment.